It is known that an internal combustion engine usually includes a cylinder head gasket, which is interposed between a cylinder head and an engine block, to seal the perimeter of the combustion chambers and of the coolant passages extending from the engine block to the cylinder head. Each combustion chamber is generally defined by an engine cylinder block, together with the top of the piston accommodated therein and a part of the cylinder head facing the piston.
As a consequence, the inner volume of the combustion chamber changes cyclically in response to the movement of the piston within the cylinder. During the movement of the piston, the pressure inside the chamber changes too. It reaches a maximum value when the fuel is ignited (maximum peak firing pressure), while it remains at lower values during the other phases of the engine cycle.
This cyclical variation of the combustion chamber inner pressure changes the gap between the cylinder head and the cylinder block. Therefore, the cylinder head gasket is subjected to pressure changes and must have permanent gap recovery capability in order to maintain satisfactory sealing-off.
The cylinder head gasket includes a stack of superimposed spring metal layers. Half and Full beads are usually provided on some of the spring-metal layers (then called functional layers) to obtain the required permanent sealing pressure. Moreover, a limiter (or stopper) protects the beads running around the respective combustion-chamber perimeter in order to make them work within a proper range of effective load. The stopper rests along the combustion-chamber edge and limits the beads closure, which follows the relative movements of the cylinder head with respect to the cylinder block (gap).
The stopper can prevents the beads from being pressed flat by the cylinder head bolts load, as it forms relief of the cylinder-head gasket, which extends along each combustion-chamber edge. In the region of the stopper, very high line load occurs due to the force exerted by the bolts, in absence of any gap between the engine block and the cylinder head. During combustion instead, the region of the stopper is typically relieved, at least momentarily, due to the pressure inside the combustion chamber that contrasts the force exerted by the bolts. This leads to high stress amplitudes in this region.
In high-performance engines, especially diesel engines, it is possible that, during maximum peak firing pressure, contact is lost in the combustion chamber perimeter sealing land and a gap between layers is formed. When contact is lost in the region of the stopper, hot combustion gases can pass, unimpeded, as far as the actual sealing-off point, which is formed by the spring-steel beads. As a result, because of the high temperature/high pressure of the combustion gases, the spring property of the bead is compromised and therefore its sealing action may be diminished.
DE10143431-A1 describes a metallic cylinder head gasket for an internal combustion engine including a cylinder block provided with adjacent combustion chambers and a cylinder head screwed to the cylinder block. The cylinder-head gasket includes two essentially planar functional layers consisting of elastic metal, in particular spring steel, between which a carrier layer made of sheet steel is arranged. On every functional layer, a full bead is provided around each combustion-chamber edge. The bead of each layer faces the other with their vertices. The beads closure is limited by at least one static stopper arranged concentrically to the respective bead.
In the region of the stopper, a support element built as a half-bead or a full-bead is provided. The support element overlies the stopper, and cannot be pressed flat. The force applied by the support element that rests directly on the stopper is in the range of about 30 to 60 N/mm under the engine operating condition (maximum peak firing pressure).
The above technology, applied to high performance engines and especially to diesel engines with aluminum cylinder block and cylinder liners, may still allow the passage of hot combustion gasses through the first sealing region of the gasket. In fact, damage of the gasket due to the leak of high temperature gas was verified (typically bore to bore), together with contamination of exhaust gasses in coolant. Moreover the support element provided on the carrier layer becomes easily damaged due to the high mechanical stress acting on it, caused by the force exerted by the cylinder head bolts.